Browning of foods is a major problem in the food and beverage industry. Browning, or oxidative darkening, can be the result of the action of an enzyme, such as polyphenol oxidase (PPO), or the result of non-enzymatic chemical reactions, for example, due to polymerization of phenolic compounds which are present in some foods. High PPO activity is present in foods which are susceptible to browning, e.g., shrimp, bananas and mushrooms. Browning causes deleterious changes in the appearance and texture of foods and beverages. Both enzymatic and non-enzymatic browning constitute serious problems for the food industry and result in millions of pounds of wasted food products per year.
Enzymatic browning, in particular, has been the subject of much research, particularly as the causative agent of shrimp melanosis, which is characterized by the formation of dark spots on shrimp. Faulkner et al., Advanced Food Research, 19: 302-310 (1953). Enzymatic browning is the result of PPO-catalyzed oxidation of mono- and diphenols to o-quinones which polymerize spontaneously to form dark-colored, high molecular weight polymers, leading to the characteristic browning or formation of dark spots.
Several methods have been developed to prevent browning, including heat inactivation of PPO and various chemical treatments, such as altering the pH of the food. Heat inactivation is not appropriate for fresh foods, such as fruits and seafood, as the high temperatures necessary to inactivate PPO change the quality and texture of the foods. Likewise, reducing the pH by adding an acid (e.g., citric acid or phosphoric acid) deleteriously affects the appearance and quality of some foods.
The control of PPO-catalyzed enzymatic browning in mushrooms using citric acid was reported by McCord and Kilara in the Journal of Food Science, 48: 1479-1483 (1983). The inhibition of polyphenol oxidase activity in an extract of Jerusalem artichokes using various sulfite compounds was described in Zawistowski et al., in Can. Inst. Food Sci. Tech. J., 20 (3): 162-164 (1987). The use of cinnamic acid, p-coumaric acid and ferulic acid to control enzymatic browning in fruit juices was described by J. R. L. Walker in Food Technology, 11: 341-345 (1976). T. C. Wong et al. report in Plant. Physiol., 48:24-30 (1971) that phloroglucinol and resorcinol, and their derivatives d-catechin and orcinol react with 4-methyl-o-quinone which is formed by PPO in peaches, although these compounds are not substrates for PPO. R. Kuttner and H. Wagreich, Arch. Biochem. Biophys., 43: 80-87 (1952) report that mushroom PPO (catecholase) is inhibited by benzoic acid and selected benzoic acid derivatives. None of these methods have proven entirely satisfactory, however, due to expense, lack of availability, or inferior performance.
Labuza in Cereal Foods World, 34(4):353 (1989) describes the use of proteases, especially ficin, in the control of enzymatic browning of certain foods. The author attributed this effect to attack on PPO by the protease.
Another method for reducing browning which has been prevalent in the food industry is adding sulfite salts to foods and beverages. Some forms of enzymatic browning, such as shrimp melanosis, have traditionally been treated by dipping or coating the shrimp or other food in a sulfite solution, such as sodium bisulfite. Sulfites are also added to wines to prevent oxidation. Sulfites reduce o-quinones to the mono- and/or diphenols, thereby inhibiting the browning reaction. However, the use of sulfites in foods has been restricted due to adverse health effects in certain individuals, and may be restricted further or even eliminated completely.